1. Field of the Invention
The present invention relates to optical fibres of improved heat resistance, and to a process for their production. The optical fibres according to the invention are obtained from a polymerisable mixture which comprises compounds containing epoxy groups, of which at least 10% by weight are siloxanes containing epoxy groups.
Optical fibres are media for information transfer.
2. Description of the Related Art
Optical fibres based on silicate glass have been known for some time. These optical fibres have very low attenuation and thus high reserves for reliable information transfer and have a high long-term service temperature. However, silicate glass fibres of this type have the disadvantage of inadequate flexibility, high costs for the production of a high-purity glass which is necessary, and complex preparation of the end faces and coupling of the end faces.
For these reasons, optical fibres based on organic polymers have recently been developed which have excellent flexibility, low weight, relatively low production costs and simpler coupling methods.
Thus, optical fibres are described (DE-OS (German Published Specification) 24 55 265; JP-B 78/42 261) which are produced from highly transparent and amorphous polymers, such as polymethyl methacrylate (PMMA), polystyrene (PS) or polycarbonate (PC). The optical fibres are produced by melting the polymer and shaping the melt, for example by extrusion. Such fibres can furthermore be provided with an outer skin, for example by coextrusion or by coating.
For production by melt forming, the polymers employed must have excellent melt-forming properties. For this reason, polymers which are unstable at the high temperatures necessary for melt forming or have high molecular weights and thus high melt viscosities or have a crosslinked structure are unsuitable as core materials.
In the melt forming process, the polymers are furthermore subjected to high temperatures and high shear forces. This can result in impairment or modification of the quality, for example due to polymer degradation, so that some of the transparency is lost. Furthermore, the risk of contamination, for example from external sources, cannot be sufficiently excluded during the melt forming process.
Optical fibres based on PMMA and PS can be produced with low attenuation. However, their heat resistance is inadequate for many applications, in particular in the machine and automotive sectors. Although optical fibres containing PC as the core material have higher heat resistance, their attenuation is relatively high due to chemical structural contributions. Polymeric plastics which have sufficiently high transparency (=low attenuation) and sufficiently high heat resistance are now being sought for the production of optical fibres for said applications. For the reasons given, thermoplastic polymers cannot fill this gap due to their low glass transition temperature.
However, optical fibres and processes for their production in which ethylenically unsaturated compounds which can be polymerised by means of free radicals are used have also been described (German Patent Specification 36 25 180). Optical fibres of this type have the advantage over those described above that they are built up from low-molecular-weight substances. These starting compounds can be produced in extremely high optical purity, for example by distillation. In general, not only monofunctional, but also difunctional or polyfunctional, ethylenically unsaturated compounds which can be polymerised by means of free radicals are employed.
Such mixtures, which contain a certain amount of a crosslinking agent, are distinguished, after the crosslinking polymerisation, by increased heat resistance. The fibres are produced by introducing the monomer mixture which can be polymerised by free radicals into the cladding material with addition of a polymerisation inhibitor and subsequently subjecting the mixture to suitable polymerisation conditions. The polymer or copolymer forming the core is thus formed in this plastic cladding.